You should use CSS for that, since it will look cleaner/better and will be easier to change later on in your life.
But, an easy way to fix that is to simply to add "width:100%" to all of the 'p' elements, that can be done through CSS by simply adding the following:
Code:
.single-post p{ width: 100%; }
Or to make it easier on you, simply add the following code into the head:
Code:
<style>
.single-post p{ width: 100%; }
</style>